Sea Service Temperature Variability in the Northwestern Hawaiian Region

Abstract

The first massive coral bleaching event reported in the Northwestern Hawaiian Islands (NWHI), the world’s second largest coral reef reserve, occurred during summer 2002. Bleaching at these remote coral atolls is believed to be related to elevated surface temperatures. The temporal and spatial variability of near-surface temperatures at the NWHI are analyzed using Reynolds Sea Surface Temperature (SST) (1 week averages, 11/1981-11/2003), and in situ temperature measured within the coral atolls (15 minute averages, 9/ 2002 – 8/2003) as part of the Northwestern Hawaiian Islands Reef Assessment and Monitoring Program (NOWRAMP). The focus is on conditions leading to high SST anomalies during the peak of the seasonal heating cycle (August-September). Comparison of Reynolds and in situ data shows reasonable correlations (0.83-0.99), regression coefficients (0.81-1.14), and residual errors (0.39-1.30°C). During summer months, mean differences between in situ and Reynolds SST do not show a consistent bias between the two instruments. These results suggest that Reynolds SST is a reasonable indicator of temperature variability at the coral atolls. Lacking in situ data before or during the 2002 bleaching event, Reynolds SST is used to characterize summer surface temperature variability at the NWHI, and to identify possible bleaching periods. Anomalously high SSTs occurred during summer 2002, but only at the three most northern atolls: Kure, Midway, and Pearl & Hermes. SSTs at the other NOWRAMP study sites were similar to long term conditions. The summer of 2002 was the only period of sustained high temperatures during the 22 year record. The intensity and duration of coral bleaching periods have been examined in terms of accumulated heat stress based on estimated thermal thresholds for bleaching at every NOWRAMP station. Again, only the 2002 event appears to have been significant, although the accumulated heat stress values barely reached 20 Degree Heating Days. Comparison with NCEP Reanalysis wind anomalies shows that the generation of high SSTs during summer 2002 is associated with a decrease in wind speeds, which occurs over short spatial scales. The v wind relaxation is caused by mid-latitude synoptic variations, which are essentially weather related with low predictability. The conclusion is that the 2002 bleaching event was caused by an unusual summer cessation of the Northeast Trades over the northern most NWHI region, causing enhanced surface heat flux and warming of the surface layer.